1. Field of Invention
The present invention relates to a method for immobilizing waste chlorides salts containing radionuclides and hazardous nuclear material for permanent disposal, and, in particular, a method for immobilizing waste chloride salts containing cesium, in a synthetic form of pollucite.
2. Description of Related Art
Electrorefining methods involving electrochemical cells are used for the recovery of fissionable materials from spent nuclear reactor fuels, including uranium and plutonium. Typically, in an electrorefining cell, an electrolyte consisting of a molten eutectic salt mixture, such as KCl and LiCl, is used to transport the metal or metals to be purified between electrode solutions. When used to treat spent nuclear reactor fuels, the salt mixture becomes contaminated with radionuclides (e.g., .sup.-137 cesium and .sup.-90 strontium), hazardous materials (e.g., barium), and other species (e.g., sodium and .sup.-129 iodine). Eventually, the salt mixtures are no longer suitable for use in the electrorefining cell.
Since the separation of cesium and strontium from the salt is difficult, the cesium and strontium, and any other radionuclides and toxic metal chlorides and iodides, are disposed along with a portion of the salt matrix. The waste salt containing the cesium and strontium is a high level waste (HLW) which must be deposited in an HLW geologic repository. To prevent an uncontrolled release of the radionuclides and other hazardous chemicals into the groundwater, the waste form must be leach resistant. Due to the very high water solubility of the waste salts, a method for encapsulating and immobilizing the waste salt is required.
The high solubility and volatility of cesium, in particular, has caused difficulties in the identification and preparation of suitable waste forms which would immobilize cesium for the necessary, extended storage time. For example, incorporation of cesium into traditional solid waste forms, including borosilicate glass, synroc, cement, or ceramics, is ineffective due to the relatively high leach rates resulting from the inherently high solubility of cesium and difficulties during processing requiring additional steps. For example, immobilizing cesium in a glass matrix involves converting the waste chloride salts into oxides or other chemical forms compatible with the glass-making process. These conversion processes are expensive and time-consuming, and restructuring the materials to form the glass requires temperatures of about 1000.degree. C., which increases the risk of volatilizing the cesium.
Ion-exchangers are an important class of materials used for the immobilization of cesium in salt solutions and molten salts. Certain cation exchange resins and various cation exchangers, such as naturally occurring and synthetic zeolites (tectosilicate mineral), are available for selectively recovering cesium from contaminated solutions. For example, zeolite matrices, including zeolite A, zeolite X, and chabazite, have been used to immobilize waste chloride salts containing cesium because of their sorption and ion exchange properties. U.S. Statutory Invention Registration H1227 discloses contacting molten waste chloride salt containing cesium with dehydrated zeolite A and maintaining the contact to allow the salt to penetrate the zeolite cavities. The salt is occluded in the zeolite and the cesium in the non-occluded salt is sorbed by ion-exchanging with the cations in the zeolite or the occluded salt, resulting in a leach resistant aluminosilicate matrix, wherein the cesium ions are present as either aluminosilicates or as occluded salt molecules. U.S. Pat. No. 4,808,318 further describes the use of a hydrated sodium phlogopite mica (phyllosilicate mineral) to recover cesium ions from waste solutions, whereby the cesium is selectively absorbed by the modified phlogopite and fixed for long-term storage.
Although methods for immobilizing cesium using ion-exchange materials can effectively purify the salt, the non-occluded surface salt must be removed from the ion-exchanger before it can be stored. In addition, if the cesium is in the form of dry, solid cesium chloride, the cesium chloride must first be dissolved in solution before being ion exchanged into a zeolite. Impurities in the solution, such as the presence of competing sodium and/or potassium ions, may also decrease the zeolite capacity for cesium. Problems are encountered in making dense, leach-resistant waste forms directly from the salt-occluded waste product, and further steps are generally required to immobilize the cesium in the ion-exchange matrix, including calcination at high temperatures or incorporation of either the cesium ions eluted from the zeolite or the zeolite containing the cesium ions into a storage medium, such as glass or cement.
A preferred method of disposing of radionuclides is by encapsulation in specific crystalline, mineral waste forms, such as sodalite and pollucite, because of their refractory properties and high resistance against leaching. Generally, these methods include mixing the radioactive ions with inorganic materials and applying heat and/or pressure to form the synthetic mineral. U.S. Pat No. 5,340,506 discloses forming a sodalite intermediate from alumina, silica, and sodium hydroxide, and mixing the sodalite intermediate with either waste salt or waste salt which has been contacted with zeolite to concentrate the radionuclides. The mixture is compacted under heat and pressure conditions to form sodalite, whereby the waste salt and radionuclides are trapped within the sodalite cage structure. U.S. Pat. No. 5,613,240 further discloses a method for producing sodalite from salt occluded zeolites by the use of heat or heat and pressure in the presence of glass. The method involves heating substantially dry zeolite, waste chloride salts, and glass to a temperature up to about 725.degree. C. to convert the zeolite to sodalite, and thereafter maintaining the sodalite at a pressure and temperature sufficient to form a sodalite product.
Pollucite (CsAlSi.sub.2 O.sub.6), a naturally occurring aluminosilicate containing cesium, is known to be one of the best crystalline waste forms for the containment of radioactive cesium, and, therefore, is a component of several mineral-based nuclear waste forms, such as synroc. Pollucite has a high loading of cesium, high thermal stability, and a high resistance to leaching.
Synthetic pollucite has been produced for immobilizing radioactive ions by aqueous hydrothermal processes and high temperature synthesis, both involving an ion exchange step. Hydrothermal transformation and recrystallization of zeolites, specifically zeolite A, zeolite X, and zeolite Y, which have been loaded with cesium in an ion exchange step, occurs in the presence of water vapor at temperatures and pressures between about 260.degree. C. to 300.degree. C. and 10 to 30 MPa, respectively. However, cesium loaded chabazite subjected to hydrothermal processes reportedly results in the formation of zeolite and not a new mineral phase, such as pollucite. Pollucite has also been produced by the hydrothermal reaction of cesium with siliceous sinter in a sodium hydroxide solution. High temperature synthesis involves subjecting ion-exchanged and cesium loaded zeolite to calcination at temperatures of about 1,200.degree. C. to produce synthetic pollucite. Dense waste forms are further produced by calcination followed by sintering or by hot pressing.
U.S. Pat. No. 5,591,420 discloses a new material: cesium titanium silicate pollucite (CsTiSi.sub.2 O.sub.6.5 or Cs.sub.2 Ti.sub.2 Si.sub.4 O.sub.13), which represents a new class of crystalline phase of Ti-containing zeolites, wherein the cages formed within the compound trap the cesium ions. The method of making the silicotitanate pollucite involves a one-step, direct thermal conversion at low temperatures (700.degree. C. to 1000.degree. C.), which minimizes the risk of volatilizing the cesium and reduces waste volumes. The cesium titanium silicate materials are made by selecting and combining proportions of cesium, silica, and titania, and heat treating the mixture. The components can be combined either by mixing oxides or carbonates of cesium, titanium, and silicon, or by synthesizing and hydrolyzing precursor materials. The resulting compounds are durable glass and ceramic materials, exhibiting low leach rates.
Problems associated with the current methods of synthesizing pollucite are high temperature requirements, which may cause volatilization and loss of the cesium, numerous process steps that increase the cost of immobilizing the cesium, and insufficient leach resistance of the storage material. In addition, cesium titanium silicate pollucite is thermally unstable and difficult to produce on a large scale.
A need exists for a method for immobilizing cesium for long term storage by incorporating the cesium into synthetic pollucite which overcomes the problems experienced in the prior art.
The present method is a simple, one step conversion process for synthesizing pollucite that eliminates the need for aqueous ion exchange and/or hydrothermal synthesis at elevated temperatures and pressures. This reduces the number of waste streams. Since the solid cesium chloride is converted to pollucite without the need to dissolve the cesium chloride in solution to perform an ion exchange step, this method can be used in conjunction with other dry processing technologies, such as pyroprocessing.
The present method for synthesizing pollucite includes mixing dry, non-aqueous cesium chloride with chabazite and heating the mixture to a temperature greater than the melting temperature of the cesium chloride (approximately 680.degree. C.), or above about 700.degree. C. The unexpected and surprising result is that pollucite forms in the presence of the chloride ion. In particular, the chloride appears to remain in the structure of the pollucite, apparently occluded as sodium chloride. The method significantly improves the rate of retention of cesium in ceramic products comprised of a salt-loaded zeolite by adding about 10% chabazite by weight to the salt-loaded zeolite prior to conversion at elevated temperatures and pressures to the ceramic composite.
Therefore, in view of the above, a basic object of the present invention is to provide a simpler method for immobilizing radioactive cesium for long term storage.
A further object of this invention is to provide a method for immobilizing radioactive cesium eliminating the need for hydrothermal synthesis which involves an aqueous ion exchange step for isolation of the cesium ion. In addition, the method is practiced at low temperatures, reducing the risk of loss of cesium.
Another object of this invention is to provide a method for immobilizing radioactive cesium by mixing non-aqueous cesium chloride with chabazite and heating the mixture to a temperature sufficient to form pollucite.
Yet another object of the invention is to provide a method for immobilizing radioactive cesium in a glass product including mixing non-aqueous cesium chloride with chabazite, heating the mixture to a temperature sufficient to form pollucite, cooling the pollucite, and heating the pollucite with glass to a temperature sufficient to form a glass pollucite product.
Yet another object of the invention is to provide a method for immobilizing radioactive cesium which includes mixing a predetermined amount of chabazite with cesium chloride and zeolite A to form a mixture of aluminosilicates, including pollucite.
Yet another object of the invention is to significantly improve the rate of retention of cesium in ceramic products comprised of a salt-loaded zeolite by adding about 10% chabazite by weight to the salt-loaded zeolite prior to its conversion at elevated temperatures and pressures to the ceramic composite.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of instrumentation and combinations particularly pointed out in the appended claims.